The following references are believed to represent the state of the art:
U.S. Pat. No. 5,765,176 to Bloomberg;
U.S. Pat. No. 7,873,183 to He, et al.;
U.S. Pat. No. 8,081,823 to Zandifar, et al.; and
U.S. Pat. No. 8,156,414 to Zandifar, et al.